2 dogs playing nicely in a small dog park / pen. No other dogs in there. One dog was a golden retriever about 70 lbs, the other looked like a wire terrier mix about 40 lbs.

Along comes a very excited GSD about 15 months about 70 lbs. She enters the arena. The GSD and Golden go into an immediate play bow, and start roughing it up. The GSD starts to get too rough, and the Golden seemed to get annoyed and correct the younger GSD.
Immediately after being "corrected", the GSD turns around and attacks the terrier, while avoiding the Golden that just brushed it back. Suddenly a bad dog fight, with the GSD getting very aggressive.

I witnessed the entire incident. Kira wasn't with me. My wife was jogging with her on the other side of the park. I stopped to say hello to the golden owner.

Why did the GSD suddenly turn aggressive on the smaller dog, after getting corrected by the Golden Retriever?